In 1905 Albert Einstein, starting from the assumption that the speed of light is the same for every observer, set out the special theory of relativity. Its consequences ran against intuition: time and length change with the observer's motion; mass and energy are interconvertible. In 1915 general relativity extended this to gravity — mass curves spacetime, and the orbits of planets are a result of that curvature.

The theory's strength lay not in its claim but in its testable predictions. During a solar eclipse in 1919, the deflection of light from distant stars in the Sun's gravity was measured and matched the prediction. Over the following century many independent observations, from GPS to gravitational waves, confirmed the theory. Relativity does not falsify Newtonian physics; it shows the limit where the latter fails, when speeds and gravity are strong.

Its impact spilled beyond physics. That a concept taken as unquestionable, like "absolute time," could change under observation made concrete the provisional and correctable nature of scientific knowledge; modern cosmology is built on this framework.